Green Roofs Mitigating the Risks of Climate Change

Taking a proactive stance, many cities are implementing green roof policies to mitigate the risks of climate change.

By Robert C. Brears*

Climate change is likely to increase the frequency and magnitude of both rainfall events and heat waves in cities around the world as global temperatures continue to rise.

Green roofs are a key aspect of mitigating urban flooding risks as they absorb rainwater and prevent water from overwhelming pipe networks and flooding streets and basements. Green roofs also reduce the heat island effect that will be magnified by heatwaves as they shade building surfaces, deflect solar radiation, and release moisture into the atmosphere, all of which reduce surrounding air temperatures.

On or after 1 January 2018, any new building with a gross floor area of 25,000 square feet or more or a building addition that causes the building to become 25,000 square feet or more, and any existing building over 25,000 square feet that is seeking a roof replacement, must incorporate a green roof. The size of the green roof is dependent on the gross floor area of the building summarized in the following Table.

Table 1. Denver’s Green Roof Requirement

Gross Floor Area (size of building)

Coverage of Available Roof Space (Size of Green Roof)

25,000-49,999f²

20%

50,000-99,999ft²

30%

100,000-149,999f²

40%

150,000-199,999f²

50%

200,000f² or greater

60%

It is possible to combine the green roof with a solar energy collection system provided that the combination is no less than 30% green roof and retains or collects for re-use at least the first .25 inches from each rainfall event or 50% of annual rainfall volume.

Amsterdam’s green roof subsidy

Owners or tenants of buildings in Amsterdam can apply for a subsidy to construct a green roof. The subsidy focuses on roof surfaces with a minimum area size of 30m² with applicants able to receive a maximum of 50% of eligible costs, up to a maximum of EUR 30- or EUR 50- per m² of green roof, depending on the water storage capacity:

A maximum of EUR 30 applies to green roofs with a water-storage capacity of 30 liters/m² or less and a maximum of EUR 50 applies to green roofs with a capacity of more than 30 liters/m²

There is a maximum per application of EUR 50,000

Melbourne’s Urban Forest Fund

Earlier in 2017, the City of Melbourne provided grant funding to businesses wishing to green their properties with various features including green roofs. The grants were matched dollar-for-dollar up to a maximum of $500,000 with eligible projects having to:

Focus on creating new green infrastructure

Demonstrate environmental and community benefits including removing air pollution and increasing community health and wellbeing

Have a clear plan for long-term maintenance

Be completed by 2019

The take-out

Cities can use a variety of policy tools including mandates, subsidies, and grants to increase the uptake of green roofs and mitigate climatic risks.